From Chabad.org: The scouts spent forty days reconnoitering the land. They returned to the Israelite encampment with specimens of Canaan’s produce, and with an ominous report. They conceded that the land flowed with milk and honey, but they warned that its population was mighty and the cities well fortified and impossible to conquer. Only Caleb and Joshua, the scouts representing the tribes of Judah and Ephraim, dissented. They argued that the land was magnificent, and there was no reason for concern because G‑d could surely bring the Israelites victory in battle. The Jewish people spent that entire night wailing, expressing their preference to return to Egypt rather than be defeated in battle by the Canaanites.
This was the ultimate test of the glass half empty/half full. While most of the spies saw negatives, the righteous ones saw those very same things as positive. For the tribal leaders to falter so mightily is staggering, but what made matters worse is that the Israelites believed the negative and ignored the truth. Is it because they trusted their leaders, and most of those leaders lead them toward the half empty side? Or was there something much worse causing all this dissension? The punishment leads one to think the latter is true.